This application claims priority of European Patent Application No. 01 810 827.4, filed on Aug. 24, 2001.
The invention relates to an artificial intervertebral disc for implantation between two vertebral bodies comprising two end plates and an elastically deformable disc pre-stressed between the end plates in the axial direction with the disc lying within a tubular, elastically deformable, fibre ring and the end plates being in tensile connection with the fibre ring.
The intervertebral disc takes on a plurality of central functions simultaneously in the vertebral column. It functions as a damper, a spacer body and also as a joint between the vertebral bodies. The demands to be made on an implant which is intended to serve as a replacement for a natural intervertebral disc in its function as an artificial intervertebral disc are correspondingly complex. For instance, the artificial intervertebral disc must naturally be made up of biocompatible materials and, as a permanent implant, must fulfil its function for the patient for life where possible. In addition to the simple function as a spacer body, the artificial intervertebral disc must in particular be able to effectively cushion the impact forces occurring in the vertebral column so that the vertebrae are not subject to excess stressxe2x80x94however, without noticeably hindering the movability of the vertebrae. A fixed connection must be ensured between the artificial intervertebral disc and the adjoining vertebra in order to suitably lead off the natural stresses of turning, tilting and shearing such as typically occur in the vertebral column.
The highest demands are thus to be made, in particular on the elastic properties of the artificial intervertebral disc, both as regards its behaviour with respect to torsional and shear stresses and with respect to pressure stresses. Overall, this means that the mechanical properties of the artificial intervertebral disc have to be reproduced as identically as possible to those of the natural intervertebral disc.
Numerous approaches are known for the replication of the natural properties of an intervertebral disc. For instance, artificial intervertebral discs are known which consist of two end plates, which are oppositely arranged, which are connected via an elastic support element, which takes over the function of an artificial nucleus and is adhesively bonded to the end plates. The artificial nucleus can, in this respect, consist of a combination of different elastic plastics which can also have a jacket. The hitherto unsolved problem of such arrangements, however, also lies in reproducing the suitable, that is the natural, very non-linear, characteristics of an intervertebral disc in the usual stress range for pressure stresses, tension stresses, shear stresses and torsional stresses in an artificial intervertebral disc.
It is therefore the object of the invention to propose an artificial intervertebral disc which reproduces the complex, elastic properties of a natural intervertebral disc as exactly as possible.
The artificial intervertebral disc which satisfies this object is characterised by the features of independent claim 1. The dependent claims relate to particularly advantageous embodiments of the invention.
The artificial intervertebral disc of the invention for implantation between two adjacent vertebral bodies comprises two end plates and an elastically deformable disc pre-stressed between the end plates in the axial direction, with the disc lying within a tubular, elastically deformable, fibre ring. The end plates are in this respect in tensile connection with the fibre ring. The disc forms a contact surface with one of the adjacent end plates which becomes larger with a compression of the artificial intervertebral disc and the elastic properties of the artificial intervertebral disc show a non-linear behaviour with increasing deformation at least with respect to a compression force.
In the artificial intervertebral disc of the invention, two opposite end plates are connected at their peripheries by means of an elastically deformable fibre ring which exerts a constant tensile force on the end plates in the unstressed state such that the disc, which is located between the two end plates within the fibre ring, is under pressure pre-stress. A substantial disadvantage of known artificial intervertebral discs lies in the fact that under the effect of an elastic deformation too long a path is stressed until the actual stress region is reached. A decisive advantage of the artificial intervertebral disc of the invention can therefore be seen in the fact that its elastic behaviour shows a non-linear behaviour at even small deformations at least with respect to compression.
The elastic behaviour of the artificial intervertebral disc of the invention is in this respect determined by the special design features of the fibre ring and the disc and in particular by the design-determined mechanical coupling of the fibre ring and the disc. For instance, the mechanical behaviour of the fibre ring with respect to elongation under tensile stress and elongation, for example, as a result of torsional stresses can already be different due to the type of fabric structure, depending on whether the fibre ring was woven, knitted, braided or manufactured in another manner. In this respect, among other things, the orientation of the fibres, of which the fibre ring is made up, in their slanting position with respect to the direction of the longitudinal axis of the artificial intervertebral disc play a roll for the forces to be transferred between the end plates. Whereas the disc alone, that is not in combination with the fibre ring, forms only a sliding friction resistance, for example with respect to pure torsional movements, a conversion of, for example, torsional movements into a compression of the disc can be achieved by the combination of both elements and an opposed slanting position of the fibres relative to the direction of the longitudinal axis of the artificial intervertebral disc. In this way, the non-linear elastic properties which the disc has with respect to a compression are transformed into the elastic torsional behaviour of the artificial intervertebral disc. The disc is under a certain pressure pre-stress via the fibre ring in the unstressed state. The possibility thus exists to adapt the elastic properties to the individual needs of a patient while observing a pre-determined geometry for the artificial intervertebral disc and without changing or replacing the materials which make up the artificial intervertebral disc. In this manner, the non-linear stress characteristics can be matched, for example, to the body weight of the patient and/or the coupling of torsional strains or other kinds of strain to a compression of the disc.
The type of construction with a fibre ring stressed by tension and a disc stressed by pressure allows the pres-stress between these two elements in the unstressed state to be selected to be so high that at extreme pressure strain, for example 4000 N, there is still a residual stress present in the tension direction which has a co-effect on a limitation of additional shear and torsional strains. Furthermore, it can be achieved (for example by the geometry of the disc in combination with a suitable choice of the pre-stress of the fibre ring) that the artificial intervertebral disc has a certain elastic neutral zone for small strains with respect to tilting and bending movements.
The materials which make up the fibre ring include biocompatible plastics to the extent that they come into contact with body tissue. The fibre ring itself can be permeable for the body""s own liquids. The end plates can be made of a metal, for example of robust titanium, or of a robust titanium alloy, which is plastically deformable for the anchoring of the fibre tube. The outer surfaces of the end plates can have zones with a metal web which facilitate the ingrowth of bone material and thus the growing together of the adjoining vertebrae with the outer surfaces of the end plates. An improvement of the anchoring between the end plate and the vertebra with respect to lateral forces is achieved by one or more projecting ribs which extend, for example, from ventral to dorsal. An additional toothed arrangement at the outer edge of the end plate, whichxe2x80x94like the projecting ribxe2x80x94does not have to be present mandatorily, can likewise substantially improve the connection between the end plate and the vertebra with respect to shear stresses. In this respect, the end plate does not necessarily have to be made up of metal, but suitable biocompatible plastics can, for example, also be considered as materials for the composition of the end plate.